In conventional PRML read channels, it is important to remove as much non-linearity as possible in the PRML read channel to achieve reliable performance. A common form of non-linearity in optical data storage is asymmetry. Asymmetry is defined as the difference between the mean of the shortest recoding patterns and the mean of the longest recording patterns in Run Length Limited (RLL) code. In the case of DVDs, asymmetry is defined as the difference between the mean of 3T peak−3T bottom and the mean of 14T peak−14T bottom. Asymmetry is also defined as the difference between a top envelope and a bottom envelope after AC-coupling. The asymmetry relies on the center of the AC coupling to be close to the center of 3T. The asymmetry is especially severe in low quality pre-embossed read only media.
Several conventional approaches have been developed to solve the asymmetry problem. In a slicer based read channel, a slicer level feedback from averaging the slicer output is commonly used. A similar implementation of slice level feedback can be found in some PRML read channel designs where active offset feedback loops are used to reduce the effect of asymmetry. In advanced PRML channel designs, more complicated adaptive Viterbi decoders are adopted.
Referring to FIG. 1, a conventional PRML read channel implementing an adaptive Vitberi decoder is shown. The circuit 10 comprises an analog-to-digital converter (ADC) 12, a partial response (PR) equalizer 14 and an adaptive Viterbi decoder 16. The ADC 12 receives analog data on a signal INPUT. The ADC 12 presents a signal SAMPLED_DATA to the PR equalizer 14. The PR equalizer presents a signal PR_OUTPUT to the Viterbi decoder 16. The Viterbi decoder 16 presents a signal DECODED_DATA. The PR equalizer 14 shapes the PRML read channel to a pre-defined target defined as 1+D+D2+D3, for instance, where D is the delay operator. The Viterbi decoder 16 implements a maximum likelihood detection method which compares an incoming sequence of digital data to all possible data sequences to calculate the error distance to each data sequence. After calculating the error distance to each data sequence, the Viterbi decoder 16 detects the path that has the minimum error distance to each data sequence.
When incoming data to the PRML channel has asymmetry, the asymmetry is also counted toward the error distances to paths in the Viterbi decoder 16. The asymmetry reduces the ability for the Viterbi decoder 16 to reject random noise. A conventional PRML read channel also includes an offset loop (not shown) and a gain loop (not shown). The offset loop and the gain loop obtains an optimal offset to generate the least amount of read error for optimizing the performance of the Viterbi decoder 16.
For the PRML read channel, the implementation of an active offset feedback loop in the PRML reader alone is not sufficient to solve the problem in a severe asymmetric case. With a severe asymmetric cure, the offset feedback loop cannot find a balance point sufficient to yield a low enough error rate for both short recorded codes and long recorded codes. The implementation of the adaptive Viterbi decoders in the PRML read channel is too complicated to be implemented in a consumer product.
It would be desirable to provide a method and/or apparatus to reduce and/or eliminate the effects of asymmetry for optical data storage with a PRML read channel.